by Will Sager
Meteor observers were recently excited about the possibility of an out-of-nowhere meteor shower, called the Tau-Herculids, that offered the possibility of a “storm” at the end of May. Many years ago, my uncle described seeing the 1966 Leonid meteor storm in Virginia and said meteors were falling too fast to count all over the sky. Later I read in Sky and Telescope about observers at Kitt Peak, who got the best display. They witnessed a rain of meteors estimated at 10-40 per second. I have been observing meteors for 45 years and have never seen such a storm. The closest I came to it was the 2001 Leonid shower, when the Earth passed through a rich stream of debris from comet Temple-Tuttle. That night, under dark skies in Arizona, I logged over 1200 over the course of the night. It was awesome, but not quite the storm that my uncle saw. Would the Tau Herculids be my storm?
Meteor showers arise from debris shed by comets or asteroids. The debris expands along the parent body orbit and if the Earth encounters that debris stream in its yearly swing around the Sun, there will be a meteor shower. Owing to perspective, the meteors seem to radiate from a single spot in the sky – like the like the snowflakes when you drive into a snowstorm (Texans will have to take my word on this). The shower is named by the location of this radiant. The Leonids from from Leo, the Perseids from Perseus, and the Tau-Herculids from a point near the star tau-Hercules. Most meteors are tiny grains, dust to sand size, and they cause streaks because they run into the atmosphere at astonishing speeds (more than 50 kilometers per second) and this causes impacted upper atmosphere atoms to be ionized and glow briefly. Every now and then, a larger fragment plows into the atmosphere, causing a bolide (aka fireball), which is an exceptionally bright meteor (usually causing the observer to hoot in delight). The most reliable annual meteor showers have debris all along the parent body orbital path, which happens to intersect the Earth’s orbit. Every year, the Earth runs through the stream, producing the annual shower.